1. Field of the Invention
The present invention relates to an optical fiber forming glass tube fabricating method using a sol-gel process, and in particular, to an apparatus and method for fabricating a tube-shaped glass monolith using a sol-gel process, which can direct a moist gel to unidirectionally dry and shrink.
2. Description of the Related Art
In general, an optical fiber preform is fabricated by inside deposition such as MCVD (Modified Chemical Vapor Deposition) or outside deposition such as VAD (Vapor phase Axial Deposition) and OVD (Outside Vapor Deposition). Among them, MCVD is most generally used in fabrication of high-quality optical fibers. In the MCVD, a preform is fabricated using a high-purity glass tube by inside deposition and overcladding. The glass tube essentially used in the MCVD is more pure and more cost-competitive when fabricated using a sol-gel process than that fabricated by other technology.
A glass tube is fabricated in a general sol-gel process as follows. A first sol is formed by dispersing fine fumed silica particles in deionized water to prevent cracking. The first sol is than gelled and the gel is then dried. The dried first gel becomes powder through grinding and classification, is thermally treated, and is re-dispersed in deionized water. Thus, a second sol is formed. The second sol is gelled in a mold, removed from the mold, dried, and then sintered to form a glass monolith.
To dry a tube-shaped moist gel in this glass tube fabrication process, the gel is first removed from a mold and dried for a long time at constant temperature and humidity. U.S. Pat. No. 5,250,096 to Allan J. Bruce, et al., entitled, Sol-Gel Method Of Making Multicomponent Glass, describes forming a first sol using a mixture of Si-alkoxide, alcohol (e.g. ethanol) and one or more metal compounds added to an appropriate liquid (alcohol if the compound is an alkoxide or an aqueous medium if the compound is a soluble salt). The first sol is gelled at 60 degrees Celsius, broken down to a fine powder and dried at a temperature between 60 and 150 degrees Celsius. The powder is redispersed in water, having its pH adjusted by the addition of an acid, to form a second sol, cast in molds of glass tubing and rubber stoppers to gel. The gel is removed from the mold, dried and sintered to form a low alkali silica glass. Other molds are discussed in U.S. Pat. No. 4,419,115 to David W. Johnson, Jr., et al., entitled Fabrication Of Sintered Hih-Silica Glasses, U.S. Pat. No. 4,786,302 to Haruo Osafune, et al., entitled, Method Of Preparing Tubular Silica Glass and U.S. Pat. No. 5,352,259 to Masato Oku, et al., entitled Method Of Manufacturing Optical Fiber Preform.
Some conventional gel drying method is limited in molding a dried gel to a thickness of 15 mm or above due to the difference between shrinkage rates of the inner and outer surfaces of the tube-shaped gel when it is dried. It is very difficult to mold the gel into a tube shape because the outer and inner surfaces of the moist gel are concurrently dried and thus develop shrinkage stresses. In addition, the moist gel is dried for a long time at constant temperature and humidity, thereby increasing fabrication cost and making it impossible to extend the gel lengthwise.
The dried gel is vulnerable to cracking even at a slight impact, and should be at least about 1 m long to mold an optical fiber forming glass tube. However, a conventional tube-shaped glass forming mold cannot relieve the gel of stresses caused by uniformless shrinkage in a lower portion of the gel due to longitudinal load of the moist gel. Hence, the dried gel is highly vulnerable to cracking, cannot be further extended lengthwise, and is unsuitable for an optical fiber forming glass tube.